In recent years there has been widespread adoption of automated reading of security documents at border entry points and other situations where the identity of a document holder requires verification. Automated reading ranges from an optical scan of OCR-readable data to the interrogation of an RFID chip within a passport or identification card, which may then involve further checking by an operator or verification by an automated system such as an e-passport gate as found in major airports. Data may also be contained in a magnetic strip or transferred wirelessly depending on the format of the document in which identity information is contained.
Optical reading of a security document is typically carried out with document readers using one or a combination of visible, infrared and ultraviolet light, depending on the information being retrieved. Often overt and covert optical security features, such as those discussed above, are included within security documents to allow the document itself to be authenticated as genuine. As discussed, covert security features may only be visible under certain illumination, such as infrared or ultraviolet light, or may, such as with an optically variable device, provide variable information when illuminated from different directions. In each case the security document is typically read by placing the document on a glass platen of a document reader, such that the information contained on the portion of the document in contact with the platen is illuminated from within the document reader. Light reflected by the document is reflected back into the reader and processed to form an image of the information (e.g. text or covert or overt security features) required. The quality of the image captured is affected greatly by the manner in which the document reflects the incident light.
A variety of security readers are known in the art. For example, U.S. Pat. No. 6,288,842, “Security Reader for Automatic Detection of Tampering and Alteration, (Mann) discloses a security reader for reading and processing information about security laminates. One example of a passport reader is commercially available from 3M Company based in St. Paul, Minn., as the 3M™ Full Page Reader.
Image enhancement by removal of unwanted reflections in image capture devices is disclosed in U.S. Pat. No. 7,136,537, “Specular Reflection in Captured Images,” (Pilu et al.). In order to remove specular reflections, two images are taken, one containing specular reflections and one where such reflections are absent. These images are blended together to create an image with reduced specular reflection, allowing underlying features to be seen. The apparatus used to achieve this effect is provided with an adjustor that is able to vary the amount of specular reflection appearing in the final image. Images are taken with one or more strobes or flashes from various directions relative to the object being imaged, and relies on each image having an absence of glare patches seen in another image. Such a method therefore takes into account reflections generated by ambient light conditions, and is not suitable for use in a document reader, for example, where illumination is well controlled and reflection features are generated by artefacts in the document being imaged, rather than artefacts generated by variations in ambient illumination.